Land was not legally subdivided into administrative units until a treaty had been concluded with the Aboriginal people ceding the land. In 1788, while part of the Province of Quebec, southern Ontario was divided into four districts: Hesse, Lunenburg, Mecklenburg, and Nassau.

In 1792, the four districts were renamed: Hesse became the Western District, Lunenburg became the Eastern District, Mecklenburg became the Midland District, and Nassau became the Home District. Counties were created within the districts.

In 1849, the districts of southern Ontario were abolished by the Province of Canada, and county governments took over certain municipal responsibilities. The Province of Canada also began creating districts in sparsely populated Northern Ontario with the establishment of Algoma District and Nipissing District in 1858.

The borders of Ontario, its new name in 1867, were provisionally expanded north and west. When the Province of Canada was formed, its borders were not entirely clear, and Ontario claimed eventually to reach all the way to the Rocky Mountains and Arctic Ocean. With Canada's acquisition of Rupert's Land, Ontario was interested in clearly defining its borders, especially since some of the new areas in which it was interested were rapidly growing. After the federal government asked Ontario to pay for construction in the new disputed area, the province asked for an elaboration on its limits, and its boundary was moved north to the 51st parallel north.

The northern and western boundaries of Ontario were in dispute after Canadian Confederation. Ontario's right to Northwestern Ontario was determined by the Judicial Committee of the Privy Council in 1884 and confirmed by the Canada (Ontario Boundary) Act, 1889 of the Parliament of the United Kingdom. By 1899, there were seven northern districts: Algoma, Manitoulin, Muskoka, Nipissing, Parry Sound, Rainy River, and Thunder Bay. Four more northern districts were created between 1907 and 1912: Cochrane, Kenora, Sudbury and Timiskaming.